Method of photoelectrolysis

ABSTRACT

A method for the photoelectrolysis of a liquid or gaseous species, comprises irradiating an ion exchange membrane of a membrane electrode assembly, wherein the membrane is an optically transparent material and comprises the species.

FIELD OF THE INVENTION

[0001] This invention relates to a method of photoelectrolysis, inparticular a method for the photoelectrolysis of water.

BACKGROUND OF THE INVENTION

[0002] A photoelectrolytic cell is one in which radiant energy causes anet chemical transformation in the cell. Of particular interest arephotoelectrolytic cells suitable for carrying out the photodissociationof water, forming hydrogen and oxygen at the cathode and anoderespectively. Water can be photodissociated using a high energy lightsource (such as a laser beam) in the presence of a catalyst (such astitanium dioxide) and a separating medium which prevents therecombination of the products.

[0003] Conventional photoelectrolytic cells are typically arranged in aplane parallel configuration, with irradiation of the electrolyteoccurring indirectly, i.e. the incident light passing through anelectrode to reach the electrolyte. Should the photoelectrolyticreaction produce a gas, the gas will generate around an electrode,bubbling out of the electrolyte. As a result of this, the entry ofadditional light is impeded. This problem has been partly addressed bymaking the electrodes transparent and introducing any catalyst as acolloidal dispersion in the electrolyte, the aim being to reduceunwanted absorption. The presence of bubbles has been accepted asinevitable.

[0004] In a photoelectrochemical cell, a current and a voltage aresimultaneously produced upon absorption of light by one or moreelectrodes. A specific type of photoelectrochemical cell is aphotovoltaic cell, which is a solid state device, usually asemiconductor such as silicon. The device absorbs photons with energiesgreater than or equal to the bandgap energy, simultaneously producingelectric power.

[0005] An electrolytic cell is one in which the input of electricalenergy results in a net chemical transformation in the cell. A commonfeature of conventional electrolytic cells is that a substantial inputof electrical energy is required to drive the electrolytic reaction at asufficient rate. This expenditure of electrical energy reduces theefficiency of the cell.

[0006] Electrochemical cells, in particular electrolytic cells, may bein the form of a membrane electrode assembly (MEA). MEAs typically havea multi-layered structure comprising (i) an Ion Exchange Membrane (IEM),(ii) a current-collecting electrode, and (iii) an electro-catalyst layeron each side.

[0007] PCT/GB02/04095 describes a composite MEA formed by an in situpolymerisation process. This Application further describes an MEA havingan improved reaction interface.

SUMMARY OF THE INVENTION

[0008] The present invention addresses the need for an efficient methodof photoelectrolysis. Photoelectolysis may be effected solely by radiantenergy, or by the simultaneous input of radiant and electrical energies.

[0009] According to the invention, a method for the photoelectrolysis ofa liquid or gaseous species comprises irradiating an ion exchangemembrane of a membrane electrode assembly. The method may furthercomprise forming an electric field across the membrane. The membrane isan optically transparent material, preferably a polymer comprising astrongly ionic group, and comprises the species. The assembly preferablycomprises a catalyst of the electrolytic reaction. Any gaseousproduct(s) may be removed by applying a pressure differential to thesystem.

[0010] A method of the invention involves the use of an MEA having anoptically transparent membrane. Judicious selection of the membranematerial may allow the direct photoelectrolysis of the species,increasing the efficiency of the operation. In particular, a method ofthe invention may be used for the photoelectrolysis of water, producinghydrogen and oxygen. This may allow cost-effective production ofhydrogen fuel from a renewable energy source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The MEA comprises an optically transparent IEM, for example aproton exchange membrane (PEM). The membrane is irradiated withhigh-energy (e.g. UV or white light) photons to yield activation of thespecies. The collision with a photon may change the state of excitationof the species. In the excited state of the species, the removal of theouter electrons, and thus the creation of residual cations, is performedmore easily. An assembly comprising a suitable catalyst may, if theirradiated photons are of sufficient energy, allow direct dissociationof water to hydrogen and oxygen without significant electrical energyinput. This may increase the efficiency of the photoelectrolyticoperation.

[0012] The membrane is preferably irradiated directly (i.e. not throughan electrode), so that any gaseous product generated at that electrodewill not interfere with the passage of light through the membrane.

[0013] Photoelectrolysis may be augmented by applying a potentialdifference across the membrane to produce an electric field. Therelative proportion of photonic and electrical input energies may bevaried. This may increase the efficiency and reduce costs such as thoseresulting from the quantity and type of catalyst used.

[0014] The membrane material is optically transparent, preferablyoptically clear. The material may be transparent to photons, e.g. highenergy, visible or UV radiation. It is preferably malleable, so that itcan formed into shapes which focus, concentrate and direct light asdesired. Thus, for example, the MEA may be in the form of a lightwaveguide or lens.

[0015] The membrane material preferably comprises a polymer whichincludes a strongly ionic group. Optically transparent polymers may beformed by the polymerisation of monomers which include monomers such ashydroxyethyl methacrylate, acrylonitrile, methyl methacrylate and/orvinyl pyrrolidone.

[0016] The material may be formed by the copolymerisation of monomerswhich include an electrically active comonomer. The electrically activecomponent can be based either upon an acid, e.g. a sulphonic acid (SO₃),phosphoric or phosphonic acid, or an alkali (OH), e.g. KOH or NaOH orammonium hydroxide. If electrically inactive comonomers are used, thematerial may be rendered electrically active by introducing stronglyionic molecules, for example using a swelling liquid technique.

[0017] The polymer is preferably hydrophilic, such that it is inherentlyable to absorb and transmit water throughout its molecular structure.Hydrophilic polymers can typically be formed by the copolymerisationfrom solution of a monomer mixture normally consisting of ahydrophobic/structural comonomer and a hydrophilic comonomer. Thepolymer is preferably cross-linked for greater stability. Cross-linkedmaterials may be formed by applying ionising radiation to the materialor by using a cross-linking agent. The use of additional cross-linkingagents allows the final water uptake to be controlled separately fromthe electrical properties.

[0018] The assembly may comprise a catalyst of the photoelectrolyticreaction. A preferred catalyst system is platinum/ruthenium depositedonto colloidal TiO₂. A desensitiser such as ruthenium (II) tris(2,2′-bipyridine)dichloride hexahydrate (Ru(bpy)₃ ²⁺), iodine or an ironcomplex with a suitable quenching compound (e.g. methyl violagen) may beused with the catalyst. Any catalyst is preferably disposed on or nearan electrode. An electrode may be translucent, transparent (e.g. a tinoxide glass) or of an “open-weave” construction, to allow thetransmission of photons through the electrode to reach the membrane. Acarbon fabric may be used as an electrode, and the fabric may beimpregnated with a layer of catalyst. The assembly may be in the form ofa stack of individual MEAs.

[0019] Where the MEA is used for the photoelectrolysis of water underalkaline conditions, preferred catalysts include, for the production ofhydrogen, Raney Ni supported on Ti or a Ni/Sn electrocatalyst(preferably of an overpotential of less than 100 mV at 0.3 Å/cm²) and,for the production of oxygen, La_(0.7)Sr_(0.3)Co_(0.8)Fe_(0.1)O₃ andLaNiO₃. Other suitable catalysts will be apparent to those of ordinaryskill in the art.

[0020] Further information regarding suitable materials and processesfor the formation of MEAs may be found in PCT/GB02/04095.

[0021] The invention will now be described by way of example only withrespect to the accompanying drawings which are each schematic views of aMEA as used in the invention.

[0022]FIG. 1 shows an MEA suitable for effecting the photoelectrolysisof water. The MEA comprises a membrane 3 disposed between electrodes 1.Disposed throughout the membrane is a titanium oxide catalyst 4.

[0023] In use, a voltage is applied between electrodes 1 using generator2, producing an electric field across the membrane 3. A container 5containing water is present at the bottom of the membrane such thatwater is absorbed into the membrane. The membrane is irradiated withhigh-energy photons 6 causing dissociation of the water to form hydrogenand oxygen at the cathode (−) and anode (+) respectively. The anode istranslucent, transparent or of an “open-weave” structure, allowingphotons to pass through it and into the membrane.

[0024]FIG. 2 is a plan view of an MEA of the invention suitable for thephotodissociation of water, the assembly arranged in a “hollow conical”geometry. The outer surfaces 7 and inner surfaces 8 of the “cone” eachcomprise an electrode/catalyst layer separated by a transparent membrane(9).

[0025] The membrane is irradiated directly at the apex of the cone, thelight path 10 focused if necessary (e.g. with a lens). Gaseous productsare generated at the inner and outer surfaces of the assembly and may beremoved (as shown by the arrows 11 and 12) by applying a pressuredifferential to the MEA.

We claim:
 1. A method for the photoelectrolysis of a liquid or gaseousspecies, which comprises irradiating an ion exchange membrane of amembrane electrode assembly, wherein the membrane is an opticallytransparent material and comprises the species.
 2. The method, accordingto claim 1, wherein the material is a polymer comprising a stronglyionic group.
 3. The method, according to claim 2, wherein the polymer ishydrophilic.
 4. The method, according to claim 2, wherein the polymer iscross-linked.
 5. The method, according to claim 1, wherein the materialis malleable.
 6. The method, according to claim 1, wherein the speciesis water.
 7. The method, according to claim 1, wherein the assembly isin the form of stack.
 8. The method, according to claim 1, which furthercomprises forming an electric field across the membrane.
 9. The method,according to claim 1, wherein the assembly comprises a catalyst.
 10. Themethod, according to claim 9, wherein the catalyst comprises platinumand ruthenium.
 11. The method, according to claim 1, wherein themembrane is irradiated directly.